1. Field of the Invention
This invention relates to electrical circuitry for detecting a combustible mixture of gas in a mine atmosphere and more particularly to an electrical combustible gas detector that includes a pair of sensor assemblies that operate continuously for a preselected time interval to generate output signals that are compared to a preselected signal and actuate an alarm system when a combustible mixture of gas is detected in the mine atmosphere.
2. Description of the Prior Art
In underground mining operations solid material is dislodged from the mine face by the operation of cutting machines and as a consequence methane gas is emitted from the rock strata. The gases may accumulate in pockets in the mine presenting a threat of explosion in the presence of a spark. Only by monitoring the concentration of the mixture of gases in the mine atmosphere is it possible to determine whether the mixture of gases has reached a critical concentration.
One method of monitoring the mixture of combustible gases in the mine atmosphere involves the use of a conventional flame safety lamp. The accuracy of a flame safety lamp, however, is dependent on the skill of the operator and only the most skilled operator can detect combustible mixtures in a concentration of less than 1% with a flame safety lamp. Even though such a concentration may not be deemed critical, detecting a combustible mixture at this level of concentration is essential in order to determine the source of the gas emission for possible accumulation to a critical level. Another disadvantage of the flame safety lamp as an indicator of combustible gas in the mine atmosphere is the difficulty in using the device within one foot of the mine face where the concentration of gases is generally the greatest and the threat of explosion the most severe. In fact, in many cases the operator is exposed to a greater hazard, such as from a roof fall, if he is required to sample the concentration of methane gas within one foot of the mine face.
Electronic devices for monitoring combustible gases are known in the art and include a single sensing head that transmits a signal through a change in electrical resistance that is proportional to the gas/air ratio. The signals are amplified and transmitted to indicator and alarm circuits which when actuated alert the operator to hazardous gas concentrations. Such devices, however, have not proved to be acceptably accurate, particularly in a mine atmosphere.
There is need for apparatus for continuously monitoring the atmosphere of a mine for the presence of a combustible mixture of gases that are emitted from the face during the mining operation. The apparatus must be capable of accuracy to below 1% concentration in air for extended intervals. The apparatus must also be adaptable for positioning at selected points in the mine, particularly at the mine face and on operating equipment.